JP3983440B2 - Cooling method of activated carbon layer in solvent recovery - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a solvent recovery method in which an organic solvent adsorbed on activated carbon is desorbed with water vapor. In the industry using an organic solvent, the organic solvent can be recovered using activated carbon adsorption and used for purification of exhaust gas.
[0002]
[Prior art]
Two or more adsorptions filled with activated carbon in the activated carbon adsorption method that recovers organic solvent contained in exhaust gas generated by various operations in various plastic products, synthetic fibers, semiconductor manufacturing processes, magnetic tape industries, etc. Adsorbing the solvent while fluidizing this activated carbon by the fixed bed type solvent recovery device that installs tanks in parallel and repeats adsorption and desorption alternately, or the raw gas sent from the tower bottom by continuously lowering the activated carbon from the tower top A fluidized bed type solvent recovery apparatus is known in which desorption is performed in a desorber separately provided with adsorbed activated carbon obtained in the lower column of the tower. The adsorption process of these apparatuses is performed under atmospheric pressure, and the desorption process is often performed under atmospheric pressure or reduced pressure using water vapor.
[0003]
Among these conventional methods, a two- tank fixed-bed type adsorption apparatus that performs atmospheric pressure desorption will be described with reference to FIG. In FIG. 4, the raw gas 1 containing an organic solvent is fed into either the adsorption tank 3-1 or 3-2 by the blower 2, and the activated carbon layer 4-1 or 4 in which the solvent in the raw gas forms a fixed bed. The exhaust gas adsorbed and purified by -2 is released to Daegu. For example, if the adsorption operation is being performed in the adsorption tank 3-1, the other adsorption tank 3-2 has finished adsorbing, switched from adsorption to desorption by the switching valve, and steam S was blown in and adsorbed. The solvent is desorbed and discharged in gaseous form along with water vapor. This mixed vapor (desorbed vapor) is led to the condensation cooler 5 where it is fully condensed by cooling. When the water of the condensate and the recovered solvent are insoluble in each other, the decanter 9 separates into a solvent phase and an aqueous phase due to the specific gravity difference, and the water is discharged and the solvent is recovered. Moreover, when both part mutually melt | dissolves mutually, it sends to a distillation tower (not shown) suitably, and separation of water and a solvent is performed.
[0004]
In this system, immediately after switching from the desorption process to the adsorption process, the inside of the adsorption tank is filled with water vapor, and the temperature in the tank is at the desorption vapor temperature (generally about 100 ° C.). (Generally around 20 to 40 ° C.), the water vapor in the cylinder is discharged, and the activated carbon layer is separated from the raw gas temperature or the adsorbed component in the raw gas by evaporation of moisture adsorbed during the desorption operation and sensible heat transfer to the raw gas. To a thermal equilibrium temperature of
For this reason, in Japanese Examined Patent Publication Nos. 53-22541 and 56-29574, when the desorption process is switched to the adsorption process, the solvent remaining in the adsorption tank at the end of the desorption process is removed from the system. In order to avoid the so-called blow-through situation where the solvent concentration increases instantaneously and shows a large value, a reflux circuit is provided to treat the gas containing the residual solvent mixed with the raw gas. Yes.
[0005]
The adsorption tank filled with activated carbon and provided with the activated carbon layer as described above is repeatedly heated and cooled between the adsorption temperature and the desorption temperature, but the heat transfer is the sensible heat of the raw gas, the adsorption / desorption of the solvent. Heat / water vapor absorption / desorption heat is performed, and a thermal dominant factor is water vapor absorption / desorption heat.
In addition, it is known that the adsorptivity of a substance deteriorates as the temperature increases.
In the case of a conventional solvent recovery using activated carbon and no cooling process, the activated carbon layer is hot at the initial stage of switching from the desorption process with water vapor to the adsorption process, and the activated carbon layer is actively cooled by evaporation of water vapor. Done. As a result, the adsorption capacity of the adsorbent is not fully demonstrated at the start of adsorption, causing the solvent in the raw gas to pass through the adsorption tank and be detected in the exhaust gas, or to generate a large amount of steam white smoke at the exhaust port. ing. In order to prevent white smoke from such an activated carbon adsorption device and prevent the solvent from blowing through at the beginning of adsorption, Japanese Patent Publication No. 63-24734 introduces a carrier gas into the adsorption tank after completion of the desorption process, There has been proposed a solvent recovery device configured to exhaust the vapor in the adsorption tank through a condenser cooler.
[0006]
Furthermore, at the beginning of adsorption, the activated carbon layer is cooled and the desorption (evaporation) of moisture occurs from the inlet side of the raw gas, resulting in a distribution difference in the amount of moisture adsorption in the activated carbon layer, and its adsorption performance such as methylene chloride and toluene. In the case of a component that is greatly affected by the water that coexists, there is a possibility that an excessive decrease in performance may occur.
[0007]
[Problems to be solved by the invention]
The present invention has been made in order to solve the above-mentioned problems. The object of the present invention is to reduce water vapor generated when moving from the desorption process to the adsorption process, and to reduce the amount of solvent in the initial stage of the adsorption process. There is a place to try to prevent the blow-through. Furthermore, it is intended to make the cooling in the activated carbon layer uniform by a simple operation and to effectively recover and develop the adsorption capacity.
[0008]
[Means for Solving the Problems]
In the present invention, an adsorption tank provided with an activated carbon layer filled with activated carbon is used to adsorb an organic solvent in the raw gas, and water is introduced into the adsorption tank under atmospheric pressure to be adsorbed on the activated carbon. In the solvent recovery method in which the desorption process for desorbing the organic solvent is repeated alternately, after the desorption process is completed, the adsorption tank is immediately depressurized to cool the activated carbon layer, and then the raw gas is introduced to restart the adsorption process. It is the cooling method of the activated carbon layer in the solvent recovery characterized. In the above-described cooling method of the activated carbon layer, it is desirable that the pressure in the adsorption tank is 0.2 atm or less.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
In order to achieve the above object, the present inventors have studied the mass transfer and thermal behavior in the adsorption tank when shifting from the desorption process to the adsorption process. Most of the exchange is heat of absorption or desorption of water vapor or moisture, and solvent blow-off at the beginning of the adsorption process is supplied with a gas containing solvent in a state where the activated carbon layer is not yet sufficiently cooled and the amount of adsorbed moisture is large. In addition, the large amount of steam white smoke generated at the beginning of the adsorption process desorbs the moisture adsorbed during the desorption operation while taking away the heat of adsorption from the activated carbon while cooling the temperature of the activated carbon layer. As a result, it was found that the partial pressure of water vapor in the exhaust gas rises and is cooled by the outside air near the discharge port to become white smoke.
[0010]
If only this is solved, a non-condensable gas such as air or nitrogen is supplied to the adsorption tank as a carrier gas as described in Japanese Patent Publication No. 63-24734, and a so-called air cooling or cooling step is provided. This can be realized by cooling the exhaust gas containing the water vapor and non-condensable gas to be discharged.
However, in this case, since the exhaust gas contains non-condensable gas, the water vapor mist generated by cooling is fine and insufficiently separated, and if the solvent remains in the activated carbon layer, Since some parts are desorbed and entrained in the exhaust gas, there may be cases where it is difficult to remove them by cooling alone. The adsorption moisture is unfavorable for the treatment of substances that have a distribution difference and the adsorption is sensitive to coexisting moisture, and part of the heat deprived from the activated carbon layer is used to raise the supply gas temperature, There was a problem that all of the heat that was used could not be used for desorption of adsorbed moisture.
[0011]
For this reason, a method for overcoming the above-mentioned problems was studied and the present invention was completed.
That is, the present invention immediately reduces the pressure in the adsorption tank after the desorption of water vapor using a vacuum generator or the like, sucks the water vapor in the space, and desorbs the moisture adsorbed on the activated carbon layer almost uniformly from within the layer. In addition, the activated carbon layer is uniformly cooled by taking the heat of adsorption of moisture into the desorbed moisture gas so as to obtain a preferable activated carbon layer temperature, and also effectively prevents white smoke from the exhaust port.
Further, in order to reduce the capacity of a vacuum generator or the like for reducing the pressure in the adsorption tank, a cooling step using a non-condensable gas such as air or nitrogen may be used in combination after the vacuum cooling.
[0012]
The degree of depressurization in the adsorption tank depends on the target cooling temperature, but the heat of adsorption of water on activated carbon is approximately 2440 kJ / kg, which is almost the same as the latent heat of vaporization of water, and the adsorption equilibrium depends on the relative humidity. Thus, the temperature of the activated carbon layer equilibrates very close to the temperature given by the vapor pressure of water. Therefore, the pressure should be reduced according to the target operating temperature, but depending on the degree of heating, in the case of atmospheric vapor desorption, the vapor temperature is set to 100 ° C. or slightly above it, so that it is considerably lower than that, Since it is desirable to make it 60 degrees C or less, it is desirable that the pressure in an adsorption tank shall be 0.2 atm or less.
Moreover, although it is thought that the lower limit of the temperature of the activated carbon layer to be cooled is essentially not present, in terms of facilities and economy, it may be considered that the normal raw gas temperature is about 20 to 40 ° C. The lower limit pressure is about 0.02 atm.
[0013]
Further, in order to reduce the capacity of the vacuum generator or the like in order to reduce the pressure in the adsorption tank, it is also effective to pass air or a gas such as nitrogen after this pressure reducing step to cool it. For example, the inside of the adsorption tank is depressurized to 0.197 atm, the activated carbon layer temperature is about 60 ° C., most of the steam is discharged and cooled, and then air is introduced to cool the activated carbon layer temperature to 30-40 ° C. To the adsorption process. As a result, the required degree of vacuum is 0.197 atm. Compared with the fact that the degree of pressure reduction for bringing the activated carbon layer temperature to 30 to 40 ° C. only by the pressure reducing operation is 0.042 to 0.073 atm, the pressure reduction is as described above. By using the operation and the subsequent passage of gas such as air or nitrogen, the degree of vacuum, that is, the vacuum pressure can be reduced.
However, in this case, since the gas supplied to cool the activated carbon layer is a non-condensable gas, it is desirable to make the capacity of the subsequent condensing system sufficient.
[0014]
Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a flowchart showing the contents of the present invention. In FIG. 1, it is assumed that the adsorption tank 3-1 is immediately after finishing the desorption process, and the adsorption tank 3-2 is in the adsorption process. In this adsorption equipment, the sum of the time required for the desorption process and the pressure reduction process is set to be equal to the time required for the adsorption process.
At this time, the adsorption tank 3-1 is filled with water vapor, and in the case of atmospheric pressure desorption, it is almost at atmospheric pressure.
The gas in the adsorption tank 3-1 in such a state is sucked through the vent pipe of the condensation cooler 5 by the vacuum pump 6, and the pressure in the adsorption tank 3-1 is reduced to a predetermined pressure. By reducing the pressure in the adsorption tank, most of the water vapor filled in the adsorption tank and the moisture adsorbed on the activated carbon during the desorption operation are desorbed to an adsorption amount corresponding to the specified pressure. The temperature of the activated carbon decreases to a temperature at which the partial pressure of water vapor equilibrates with the partial pressure of water vapor.
Since the vacuum pump to be used is almost a condensable gas, it may be of a small capacity by being installed downstream of the condenser cooler. The decrease in the temperature in the adsorption tank follows the decrease in the pressure in the adsorption tank, and since the purpose is cooling the activated carbon layer, it is sufficient to reach a predetermined temperature, and it is not necessary to lengthen the holding time. The time for the decompression step is approximately 5 to 20 minutes, although it depends on the capacity of the vacuum pump. Since the temperature of the activated carbon layer is lowered by utilizing desorption of adsorbed water due to reduced pressure, the temperature of the activated carbon layer is uniformly reduced.
Thereafter, the raw gas is introduced into the adsorption tank 3-1, and the process proceeds to the adsorption process. At the same time, the adsorption tank 3-2 proceeds to the desorption process by blowing water vapor. Since the adsorption tank 3-1 is sufficiently cooled by the decompression process and then moves to the adsorption process, the solvent in the adsorption gas is not adsorbed and is not released into the exhaust gas.
[0015]
【Example】
Using an experimental device in which activated carbon is packed in a column with a diameter of 40 mm at a height of 500 mm, normal pressure water vapor at 100 ° C. assuming water vapor desorption is sufficiently passed to reach a predetermined temperature. The change in the temperature of the activated carbon layer was measured when cooling was performed at a temperature of [° C.] and when cooling was performed according to the reduced pressure (0.042 atm) of the present invention, and the results are shown in FIG. In addition, the temperature measurement at this time is 50 mm (lower layer) from the bottom surface of the activated carbon layer, 250 mm (middle layer) from the bottom surface of the activated carbon layer, and 450 mm (upper layer) from the bottom surface of the activated carbon layer. went.
As shown in FIG. 2b, which shows the result of the conventional method of the comparative example, in the case of air cooling, in the lower layer (♦ mark) on the air inlet side, the temperature rapidly decreases, but the layer on the air outlet side It can be seen that in the upper stage (▲ mark), the decrease is gentle, and it takes a long time to cool the whole to the inlet temperature. On the other hand, in the case of the reduced pressure cooling of the present invention (FIG. 2a), the temperature was uniformly lowered within a few minutes. Furthermore, from the temperature change of these activated carbon layers, the temperature drops rapidly and uniformly in the present invention, but in the case of conventional air cooling, it is recognized that the residual moisture in the activated carbon layer varies depending on the site, and the adsorption There is a risk of adverse effects when changing to operation.
[0016]
Next, the solvent was recovered from the methylene chloride and ethanol solvent-containing gas using the adsorption facility based on FIG. The time for each process is 90 minutes for the adsorption process, 75 minutes for the desorption process using water vapor, and 15 minutes for the vacuum cooling process. In the reduced pressure cooling process, the pressure was reduced to 0.056 atm corresponding to the raw gas temperature of 35 ° C. The total organic substance concentration in the exhaust gas during the adsorption operation in this repetitive operation was measured using a hydrocarbon meter (THC meter), and is shown in FIG. 3 in comparison with a conventional method that does not use a vacuum cooling step. . In the case of the conventional method, about 200 to 500 ppm of the adsorbed component solvent was detected in the exhaust gas immediately after switching to the adsorption step, but in the case of the present invention, it was below the detection lower limit. In addition, a large amount of white smoke that had been generated at the exhaust port at the beginning of the adsorption process was not observed.
[0017]
【The invention's effect】
According to the present invention, it is possible to prevent the solvent from blowing through at the initial stage of switching to the adsorption process, and to prevent the generation of a large amount of steam white smoke at the exhaust port at this time.
If the purpose is simply to cool the adsorption tank, air can be passed at room temperature without using reduced pressure, but the supply gas has an effect as a carrier and the temperature of the activated carbon layer is still at the initial stage of cooling. When there is a high temperature, there is a possibility that the residual solvent in the coal bed may be desorbed and transported, and since this gas contains a non-condensed gas, it is difficult to prevent white smoke due to water vapor unless sufficient cooling is performed. Further, if the solvent component is contained, the solvent always has a gas partial pressure, and thus complete removal and recovery is difficult. According to the reduced-pressure cooling system of the present invention, the generated gas (vapor) is condensable and can be easily removed.
In addition, when the solvent to be recovered is affected by the coexisting moisture, such as methylene chloride, the control of residual moisture in the activated carbon layer is important for maintaining the adsorption performance. By cooling, it is possible to uniformly desorb moisture adsorbed during the desorption operation.
In addition, it is possible to use an operation such as air cooling together with a pressure reducing operation in order to reduce the load of the vacuum generator used for pressure reduction. In this case, it is preferable to perform the air cooling operation after the pressure reducing operation, The water vapor is extracted under reduced pressure, and then cooled to a more appropriate temperature with a non-condensable gas such as air at room temperature. Although the effect is slightly inferior to the case of cooling to the target temperature by the reduced pressure cooling alone, the effect can be selected in consideration of the equipment cost, the operation cost and the like.
[Brief description of the drawings]
FIG. 1 shows a solvent recovery flow chart of a reduced pressure cooling operation method according to the present invention. FIG. 2 shows an example of a result of measuring a temperature change in an activated carbon layer by reduced pressure cooling according to the present invention. Example of change measurement results [Fig. 3] Hydrocarbon concentration in exhaust gas during adsorption operation according to the method of the present invention and the conventional method [Fig. 4] Flow diagram of solvent recovery equipment by the conventional method [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Raw gas 2 Gas blower 3-1, 3-2 Adsorption layer 4-1, 4-2 Activated carbon layer 5 Condensation cooler 6 Vacuum pump 7 Relay layer 8 Feed pump 9 Decanter S Steam (steam)
W Cooling water

Claims (3)

Using an adsorption tank equipped with an activated carbon layer filled with activated carbon, an adsorption process for adsorbing the organic solvent in the raw gas and water vapor introduced into the adsorption tank under atmospheric pressure to desorb the organic solvent adsorbed on the activated carbon In the solvent recovery method in which the desorption process is alternately repeated, after the desorption process is completed, the adsorption tank is immediately depressurized to cool the activated carbon layer, and then the raw gas is introduced to restart the adsorption process. Cooling method of activated carbon layer in recovery. The method for cooling an activated carbon layer in solvent recovery according to claim 1, wherein the pressure in the adsorption tank is 0.2 atm or less. The method for cooling an activated carbon layer in solvent recovery according to claim 1 or 2, wherein air or nitrogen is blown after the activated carbon layer is cooled by immediately reducing the pressure in the adsorption tank after completion of the desorption process.

Images